- Pike Programming Language

Text version

Internet Engineering Task Force (IETF)                       J. Peterson
Request for Comments: 8946                                       Neustar
Updates: 8224                                              February 2021
Category: Standards Track                                               
ISSN: 2070-1721

Personal Assertion Token (PASSporT) Extension for Diverted Calls

Abstract

The Personal Assertion Token (PASSporT) is specified in RFC 8225 to convey cryptographically signed information about the people involved in personal communications. This document extends PASSporT to include an indication that a call has been diverted from its original destination to a new one. This information can greatly improve the decisions made by verification services in call forwarding scenarios. Also specified here is an encapsulation mechanism for nesting a PASSporT within another PASSporT that assists relying parties in some diversion scenarios.

This document updates RFC 8224.

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 7841.

Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at https://www.rfc-editor.org/info/rfc8946.

Copyright Notice

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

   1.  Introduction
   2.  Terminology
   3.  The "div" PASSporT Type and Claim
   4.  Using "div" in SIP
     4.1.  Authentication Service Behavior
     4.2.  Verification Service Behavior
   5.  The "div-o" PASSporT Type
     5.1.  Processing "div-o" PASSporTs
   6.  Definition of "opt"
   7.  "div" and Redirection
   8.  Extending "div" to Work with Service Logic Tracking
   9.  IANA Considerations
     9.1.  JSON Web Token Claims Registrations
       9.1.1.  "div" registration
       9.1.2.  "opt" registration
     9.2.  PASSporT Type Registrations
   10. Privacy Considerations
   11. Security Considerations
   12. References
     12.1.  Normative References
     12.2.  Informative References
   Appendix A.  Keys for Examples
   Acknowledgments
   Author's Address

1. Introduction

A Personal Assertion Token (PASSporT) [RFC8225] is a token format based on the JSON Web Token (JWT) [RFC7519] for conveying cryptographically signed information about the people involved in personal communications; it is used by the Secure Telephone Identity Revisited (STIR) protocol [RFC8224] to convey a signed assertion of the identity of the participants in real-time communications established via a protocol like SIP. This specification extends PASSporT to include an indication that a call has been diverted from its original destination to a new one.

Although the STIR problem statement [RFC7340] is focused on preventing the impersonation of the caller's identity, which is a common enabler for threats such as robocalling and voicemail hacking on the telephone network today, it also provides a signature over the called number at the time that the authentication service sees it. As [RFC8224], Section 12.1 describes, this protection over the contents of the To header field is intended to prevent a class of cut-and-paste attacks. If Alice calls Bob, for example, Bob might attempt to cut and paste the Identity header field in Alice's INVITE into a new INVITE that Bob sends to Carol, and thus be able to fool Carol into thinking the call came from Alice and not Bob. With the signature over the To header field value, the INVITE Carol sees will clearly have been destined originally for Bob, and thus Carol can view the INVITE as suspect.

However, as [RFC8224], Section 12.1.1 points out, it is difficult for Carol to confirm or reject these suspicions based on the information she receives from the baseline PASSporT object. The common "call forwarding" service serves as a good example of the reality that the original called party number is not always the number to which a call is delivered. There are a number of potential ways for intermediaries to indicate that such a forwarding operating has taken place. The address in the To header field value of SIP requests is not supposed to change, according to baseline SIP behavior [RFC3261]; instead, it is the Request-URI that is supposed to be updated when a call is retargeted. Practically speaking, however, many operational environments do alter the To header field. The History-Info header field [RFC7044] was created to store the Request-URIs that are discarded by a call in transit. The SIP Diversion header field [RFC5806], though historic, is still used for this purpose by some operators today. Neither of these header fields provide any cryptographic assurance of secure redirection, and they both record entries for minor syntactical changes in URIs that do not reflect a change to the actual target of a call.

Therefore, this specification extends PASSporT with an explicit indication that the original called number in PASSporT no longer reflects the destination to which a call is intended to be delivered. For this purpose, it specifies a Divert PASSporT type ("div") for use in common SIP retargeting cases; it is expected that in this case, SIP INVITE requests will carry multiple Identity header fields, each containing its own PASSporT. Throughout this document, PASSporTs that contain a "div" element will be referred to as "div" PASSporTs. Verification services and the relying parties who make authorization decisions about communications may use this diversion indication to confirm that a legitimate retargeting of the call has taken place, rather than a cut-and-paste attack. For out-of-band use cases [RFC8816] and other non-SIP applications of PASSporT, a separate "div-o" PASSporT type is also specified, which defines an "opt" PASSporT element for carrying nested PASSporTs within a PASSporT. These shall in turn be referred to in this document as "div-o" PASSporTs.

2. Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.

3. The "div" PASSporT Type and Claim

This specification defines a PASSporT [RFC8225] type called "div", which may be employed by authentication services located at retargeting entities. All "div" PASSporTs MUST contain a new JSON Web Token "div" claim, also specified in this document, which indicates a previous destination for a call during its routing process. When a retargeting entity receives a call signed with a PASSporT, it may act as an authentication service and create a new PASSporT containing the "div" claim to attach to the call.

Note that a new PASSporT is only necessary when the canonical form of the "dest" identifier (per the canonicalization procedures in [RFC8224], Section 8.3) changes due to this retargeting. If the canonical form of the "dest" identifier is not changed during retargeting, then a new PASSporT with a "div" claim MUST NOT be produced.

The headers of the new PASSporTs generated by retargeting entities MUST include the "div" PASSporT type and an "x5u" field pointing to a credential that the retargeting entity controls. "div" PASSporTs MUST use full form instead of compact form. The new PASSporT header will look as follows:

{ "typ":"passport",

     "ppt":"div",
     "alg":"ES256",
     "x5u":"https://www.example.com/cert.cer" }

A "div" PASSporT claims set is populated with elements drawn from the PASSporT(s) received for a call by the retargeting entity; at a high level, the original identifier for the called party in the "dest" object will become the "div" claim in the new PASSporT. If the "dest" object of the original PASSporT contains multiple identifiers, because it contains one or more name/value pairs with an array as its value, the retargeting entity MUST select only one identifier from the value(s) of the "dest" object to occupy the value of the "div" field in the new PASSporT. Moreover, it MUST select an identifier that is within the scope of the credential that the retargeting entity will specify in the "x5u" of the PASSporT header (as described below).

The new target for the call selected by the retargeting entity becomes the value of the "dest" object of the new PASSporT. The "orig" object MUST be copied into the new PASSporT from the original PASSporT received by the retargeting entity. The retargeting entity SHOULD retain the "iat" object from the original PASSporT, though if in the underlying signaling protocol (e.g., SIP) the retargeting entity changes the date and time information in the retargeted request, the new PASSporT should instead reflect that date and time. No other claims or extensions are to be copied from the original PASSporT to the "div" PASSporT.

So, for an original PASSporT claims set of the form:

      { "dest":{"tn":["12155551213"]},
        "iat":1443208345,
        "orig":{"tn":"12155551212"} }

If the retargeting entity is changing the target from 12155551213 to 12155551214, the claims set of a "div" PASSporT generated by the retargeting entity would look as follows:

      { "dest":{"tn":["12155551214"]},
        "div":{"tn":"121555551213"},
        "iat":1443208345,
        "orig":{"tn":"12155551212"} }

The combined full form PASSporT (with a signature covered by the ES256 keys given in Appendix A) would look as follows:

eyJhbGciOiJFUzI1NiIsInBwdCI6ImRpdiIsInR5cCI6InBhc3Nwb3J0IiwieDV1Ij \ oiaHR0cHM6Ly93d3cuZXhhbXBsZS5jb20vY2VydC5jZXIifQ.eyJkZXN0Ijp7InRuI \ jpbIjEyMTU1NTUxMjE0Il19LCJkaXYiOnsidG4iOiIxMjE1NTU1NTEyMTMifSwiaWF \ 0IjoxNDQzMjA4MzQ1LCJvcmlnIjp7InRuIjoiMTIxNTU1NTEyMTIifX0.xBHWipDEE \ J8a6TsdX6xUXAnblsFiGUiAxwLiv0HLC9IICj6eG9jQd6WzeSSjHRBwxmChHhVIiMT \ SqIlk3yCNkg

The same "div" PASSporT would result if the "dest" object of the original PASSporT contained an array value, such as {"tn":["12155551213","19995551234"]}, and the retargeting entity chose to retarget from the first telephone number in the array. Every "div" PASSporT is diverting from only one identifier.

Note that the "div" element may contain other name/value pairs than just a destination, including a History-Info indicator (see Section 8). After the PASSporT header and claims have been constructed, their signature is generated per the guidance in [RFC8225] -- except for the credential required to sign it. While in the ordinary construction of a PASSporT, the credential used to sign will have authority over the identity in the "orig" claim (for example, a certificate with authority over the telephone number in "orig" per [RFC8226]), for all PASSporTs using the "div" type the signature MUST be created with a credential with authority over the identity present in the "div" claim. So for the example above, where the original "dest" is "12155551213", the signer of the new PASSporT object MUST have authority over that telephone number and need not have any authority over the telephone number present in the "orig" claim.

Note that Identity header fields are not ordered in a SIP request, and in a case where there is a multiplicity of Identity header fields in a request, some sorting may be required to match "div" PASSporTs to their originals.

PASSporTs of type "div" MUST NOT contain an "opt" (see Section 6) element in their payload.

4. Using "div" in SIP

This section specifies SIP-specific usage for the "div" PASSporT type and its handling in the SIP Identity header field "ppt" parameter value. Other protocols using PASSporT may define behavior specific to their use of the "div" claim.

4.1. Authentication Service Behavior

An authentication service only adds an Identity header field value containing the "div" PASSporT type to a SIP request that already contains at least one Identity header field value; it MUST NOT add a "div" PASSporT to an INVITE that contains no Identity header field. The retargeting entity SHOULD act as a verification service and validate the existing Identity header field value(s) in the request before proceeding; in some high-volume environments, it may instead put that burden of validating the chain entirely on the terminating verification service. As the authentication service will be adding a new PASSporT that refers to an original, it MUST NOT remove the original request's Identity header field value before forwarding.

As was stated in Section 3, the authentication service MUST sign any "div" PASSporT with a credential that has a scope of authority covering the identity it populates in the "div" element value. Note that this is a significant departure from baseline STIR authentication service behavior, in which the PASSporT is signed by a credential with authority over the "orig" field. The "div" value reflects the URI that caused the call to be routed to the retargeting entity, so in ordinary operations, it would already be the STIR entity holding the appropriate private keying material for calls originating from that identity.

A SIP authentication service typically will derive the "dest" element value of a "div" PASSporT from a new Request-URI that is set for the SIP request before it is forwarded. Older values of the Request-URI may appear in header fields like Diversion or History-Info; this document specifies an optional interaction with History-Info below in Section 8. Note as well that because PASSporT operates on canonicalized telephone numbers and normalized URIs, many smaller changes to the syntax of identifiers that might be captured by other mechanisms that record retargeting (like History-Info) will likely not require a "div" PASSporT.

When adding an Identity header field with a PASSporT claims set containing a "div" claim, SIP authentication services MUST also add a "ppt" parameter to that Identity header with a value of "div". For the example PASSporT given in Section 3, the new Identity header added after retargeting might look as follows:

   Identity:eyJhbGciOiJFUzI1NiIsInBwdCI6ImRpdiIsInR5cCI6InBhc3Nwb3J0I \
   iwieDV1IjoiaHR0cHM6Ly93d3cuZXhhbXBsZS5jb20vY2VydC5jZXIifQ.eyJkZXN0 \
   Ijp7InRuIjpbIjEyMTU1NTUxMjE0Il19LCJkaXYiOnsidG4iOiIxMjE1NTU1NTEyMT \
   MifSwiaWF0IjoxNDQzMjA4MzQ1LCJvcmlnIjp7InRuIjoiMTIxNTU1NTEyMTIifX0. \
   xBHWipDEEJ8a6TsdX6xUXAnblsFiGUiAxwLiv0HLC9IICj6eG9jQd6WzeSSjHRBwxm \
   ChHhVIiMTSqIlk3yCNkg; \
   info=<https://www.example.com/cert.cer>;ppt="div"

Note that in some deployments, an authentication service will need to generate "div" PASSporTs for a request that contains multiple non-"div" Identity header field values. For example, a request arriving at a retargeting entity might contain, in different Identity header fields, a baseline [RFC8224] PASSporT and a PASSporT of type "rph" [RFC8443] signed by a separate authority. Provided that these PASSporTs share the same "orig" and "dest" values, the retargeting entity's authentication service SHOULD generate only one "div" PASSporT. If the "orig" or "dest" of these PASSporTs differ, however, one "div" PASSporT SHOULD be generated for each non-"div" PASSporT. Note that this effectively creates multiple chains of "div" PASSporTs in a single request, which complicates the procedures that need to be performed at verification services.

Furthermore, a request may also be retargeted a second time, at which point the subsequent retargeting entity SHOULD generate one "div" PASSporT for each previous "div" PASSporT in the request that contains a "dest" object with the value of the current target -- but not for "div" PASSporTs with earlier targets. Ordinarily, the current target will be readily identifiable, as it will be in the last "div" PASSporT in each chain, and in SIP cases, it will correspond to the Request-URI received by the retargeting entity. Moreover, the current target will be an identifier that the retargeting entity possesses a credential to sign for, which may not be true for earlier targets. Ultimately, on each retargeting, the number of PASSporTs added to a request will be equal to the number of non-"div" PASSporTs that do not share the same "orig" and "dest" object values.

4.2. Verification Service Behavior

[RFC8224], Section 6.2, Step 5 requires that specifications defining "ppt" values describe any additional or alternative verifier behavior. The job of a SIP verification service handling one or more "div" PASSporTs is very different from that of a traditional verification service. At a high level, the immediate responsibility of the verification service is to extract all PASSporTs from the two or more Identity header fields in a request, identify which are "div" PASSporTs and which are not, and then order and link the "div" PASSporTs to the original PASSporT(s) in order to build one or more chains of retargeting.

In order to validate a SIP request using the "div" PASSporT type, a verification service needs to inspect all of the valid Identity header field values associated with a request, as an Identity header field value containing "div" necessarily refers to an earlier PASSporT already in the message. For each "div" PASSporT, the verification service MUST find an earlier PASSporT that contains a "dest" claim with a value equivalent to the "div" claim in each "div" PASSporT. It is possible that this earlier PASSporT will also contain a "div" and that it will in turn chain to a still earlier PASSporT stored in a different Identity header field value. If a complete chain cannot be constructed, the verification service cannot complete "div" validation; it MAY still validate any non-"div" PASSporTs in the request per the normal procedures in [RFC8224]. If a chain has been successfully constructed, the verification service extracts from the outermost (that is, the most recent) PASSporT in the chain a "dest" field; this will be a "div" PASSporT that no other "div" PASSporT in the SIP request refers to. Its "dest" element value will be referred to in the procedures that follow as the value of the "outermost "dest" field".

Ultimately, by looking at this chain of transformations and validating the associated signatures, the verification service will be able to ascertain that the appropriate parties were responsible for the retargeting of the call to its current destination. This can help the verification service to determine that the original PASSporT in the call was not simply used in a cut-and-paste attack and inform any associated authorization decisions in terms of how the call will be treated -- though, per [RFC8224], Section 6.2.1, that decision is a matter of local policy and is thus outside the scope of this specification.

A verification service parses a chain of PASSporTs as follows:

The verification service MUST compare the value in the outermost "dest" field to the target of the call. As it is anticipated that SIP authentication services that create "div" PASSporTs will populate the "dest" header from the retargeted Request-URI (see Section 4.1), in ordinary SIP operations, the Request-URI is where verification services will find the latest call target. Note, however, that after a "div" PASSporT has been added to a SIP request, the Request-URI may have been updated during normal call processing to an identifier that no longer contains the logical destination of a call; in this case, the verification service MAY compare the "dest" field to a provisioned telephone number for the recipient.

The verification service MUST validate the signature over the outermost "div" PASSporT and establish that the credential that signed the "div" PASSporT has the authority to attest for the identifier in the "div" element of the PASSporT (per [RFC8224], Section 6.2, Step 3).

The verification service MUST validate that the "orig" field of the innermost PASSporT of the chain (the only PASSporT in the chain that will not be of PASSporT type "div") is equivalent to the "orig" field of the outermost "div" PASSporT; in other words, that the original calling identifier has not been altered by retargeting authentication services. If the "orig" value has changed, the verification service MUST treat the entire PASSporT chain as invalid. The verification service MUST also verify that all other "div" PASSporTs in the chain share the same "orig" value. Then, the verification service validates the relationship of the "orig" field to the SIP-level call signaling per the guidance in [RFC8224], Section 6.2, Step 2.

The verification service MUST check the date freshness in the outermost "div" PASSporT, per [RFC8224], Section 6.2, Step 4. Furthermore, it is RECOMMENDED that the verification service check that the "iat" field of the innermost PASSporT is also within the date freshness interval; otherwise, the verification service could allow attackers to replay an old, stale PASSporT embedded in a fresh "div". However, note that in some use cases, including certain ways that call transfers are implemented, it is possible that an established call will be retargeted long after it has originally been placed, and verification services may want to allow a longer window for the freshness of the innermost PASSporT if the call is transferred from a trusted party (as an upper bound, a freshness window on the order of three hours might suffice).

The verification service MUST inspect and validate the signatures on each and every PASSporT object in the chain between the outermost "div" PASSporT and the innermost PASSporT. Note that (per Section 4.1) a chain may terminate at more than one innermost PASSporT, in cases where a single "div" is used to retarget from multiple innermost PASSporTs. Also note that [RFC8224], Section 6.2, Step 1 applies to the chain validation process; if the innermost PASSporT contains an unsupported "ppt", its chain MUST be ignored.

Note that the To header field is not used in the first step above. Optionally, the verification service MAY verify that the To header field value of the received SIP signaling is equal to the "dest" value in the innermost PASSporT; however, as has been observed in some deployments, the original To header field value may be altered by intermediaries to reflect changes of target. Deployments that change the original To header field value to conceal the original destination of the call from the ultimate recipient should note that the original destination of a call may be preserved in the innermost PASSporT. Future work on "div" might explore methods to implement that sort of policy while retaining a secure chain of redirection.

5. The "div-o" PASSporT Type

This specification defines a "div-o" PASSporT type that uses the "div" claim element in conjunction with the "opt" (Section 6) claim element. As is the case with "div" PASSporT type, a "div-o" PASSporT is created by an authentication service acting for a retargeting entity, but instead of generating a separate "div" PASSporT to be conveyed alongside an original PASSporT, the authentication service in this case embeds the original PASSporT inside the "opt" element of the "div-o" PASSporT. The "div-o" extension is designed for use in non-SIP or gatewayed SIP environments where the conveyance of PASSporTs in separate Identity header fields in impossible, such as out-of-band STIR scenarios [RFC8816].

The syntax of "div-o" PASSporTs is very similar to "div". A "div-o" PASSporT header object might look as follows:

{ "typ":"passport",

     "ppt":"div-o",
     "alg":"ES256",
     "x5u":"https://www.example.com/cert.cer" }

Whereas a "div" PASSporT claims set contains only the "orig", "dest", "iat", and "div" elements, the "div-o" additionally MUST contain an "opt" element (see Section 6), which encapsulates the full form of the previous PASSporT from which the call was retargeted, triggering the generation of this "div-o". The format of the "opt" element is identical to the encoded PASSporT format given in Appendix A of [RFC8225].

So, for an original PASSporT claims set of the form:

      { "orig":{"tn":"12155551212"},
        "dest":{"tn":["12155551213"]},
        "iat":1443208345 }

If the retargeting entity is changing the target from 12155551213 to 12155551214, the new PASSporT claims set for "div-o" would look as follows:

    { "orig":{"tn":"12155551212"},
    
      "dest":{"tn":["12155551214"]},
      "iat":1443208345,
      "div":{"tn":"121555551213"},
      "opt":"eyJhbGciOiJFUzI1NiIsInR5cCI6InBhc3Nwb3J0IiwieDV1IjoiaHR0c \
      HM6Ly93d3cuZXhhbXBsZS5jb20vY2VydC5jZXIifQ.eyJkZXN0Ijp7InRuIjpbIj \
      EyMTU1NTUxMjEzIl19LCJpYXQiOjE0NDMyMDgzNDUsIm9yaWciOnsidG4iOiIxMj \
      E1NTU1MTIxMiJ9fQ.1bEzkzcNbKvgz4QoMx0_DJ2T8qFMDC1sPqHPXl1WvbauzRJ \
      RvYlZqQ0qgGTlS8tJ_wXjVe07Z3wvDrdApHhhYw" }

While in ordinary operations, it is not expected that SIP would carry a "div-o" PASSporT, it might be possible in some gatewaying scenarios. The resulting full form Identity header field with a "div-o" PASSporT would look as follows:

   Identity:eyJhbGciOiJFUzI1NiIsInBwdCI6ImRpdi1vIiwidHlwIjoicGFzc3Bvc \
   nQiLCJ4NXUiOiJodHRwczovL3d3dy5leGFtcGxlLmNvbS9jZXJ0LmNlciJ9.eyJkZX \
   N0Ijp7InRuIjoiMTIxNTU1NTEyMTQifSwiZGl2Ijp7InRuIjoiMTIxNTU1NTUxMjEz \
   In0sImlhdCI6MTQ0MzIwODM0NSwib3B0IjoiZXlKaGJHY2lPaUpGVXpJMU5pSXNJbl \
   I1Y0NJNkluQmhjM053YjNKMElpd2llRFYxSWpvaWFIUjBjSE02THk5M2QzY3VaWGho \
   YlhCc1pTNWpiMjB2WTJWeWRDNWpaWElpZlEuZXlKa1pYTjBJanA3SW5SdUlqcGJJak \
   V5TVRVMU5UVXhNakV6SWwxOUxDSnBZWFFpT2pFME5ETXlNRGd6TkRVc0ltOXlhV2Np \
   T25zaWRHNGlPaUl4TWpFMU5UVTFNVEl4TWlKOWZRLjFiRXpremNOYkt2Z3o0UW9NeD \
   BfREoyVDhxRk1EQzFzUHFIUFhsMVd2YmF1elJKUnZZbFpxUTBxZ0dUbFM4dEpfd1hq \
   VmUwN1ozd3ZEcmRBcEhoaFl3Iiwib3JpZyI6eyJ0biI6IjEyMTU1NTUxMjEyIn19.C \
   HeA9wRnthl7paMe6rP0TARpmFCXjmi_vF_HRz2O_oulB_R-G9xZNiLVvmvHv4gk6LI \
   LaDV2y2VtHTLIEgmHig; \
   info=<https://www.example.com/cert.cer>;ppt="div-o"

5.1. Processing "div-o" PASSporTs

The authentication and verification service procedures required for "div-o" closely follow the guidance given in Sections 4.1 and 4.2, with the major caveats being, first, that they do store or retrieve PASSporTs via the Identity header field values of SIP requests and, second, that they process nested PASSporTs in the "opt" claim element. But transposing the rest of the behaviors described above to creating and validating "div-o" PASSporTs is straightforward.

For the "div-o" PASSporT type, retargeting authentication services that handle calls with one or more existing PASSporTs will create a corresponding "div-o" PASSporT for each received PASSporT. Each "div-o" PASSporT MUST contain an "opt" claim set element with the value of the original PASSporT from which the "div-o" was created; as specified in Section 4.1, the authentication service MUST populate the "div" claim set element of the "div-o" PASSporT with the "dest" field of the original PASSporT. Each received PASSporT may in turn contain its own "opt" claim set element if the retargeting authentication service is not the first in its chain. Note that if the retargeting authentication service is handling a call with multiple PASSporTs, which in ordinary SIP operation would result in the construction of multiple "div" chains, it will in effect be generating one "div-o" PASSporT per chain.

The job of a verification service is in many ways easier for "div-o" than for "div", as the verification service has no need to correlate the PASSporTs it receives and assemble them into chains, as any chains in "div-o" will be nested through the "opt" element. Nonetheless, the verification services MUST perform the same chain validation described in Section 4.2 to validate that each nested PASSporT shares the same "orig" field as its enclosing PASSporT and that the "dest" field of each nested PASSporT corresponds to the "div" field of its enclosing PASSporT. The same checks MUST also be performed for freshness, signature validation, and so on. It is similarly OPTIONAL for the verification service to determine that the "dest" claims element of the outermost PASSporT corresponds to the called party indication of receive telephone signaling, where such indication would vary depending on the using protocol.

How authentication services or verification services receive or transport PASSporTs for "div-o" is outside the scope of this document and dependent on the using protocol.

6. Definition of "opt"

The presence of an "Original PASSporT" ("opt") claims set element signifies that a PASSporT encapsulates another entire PASSporT within it, typically a PASSporT that was transformed in some way to create the current PASSporT. Relying parties may need to consult the encapsulated PASSporT in order to validate the identity of a caller. "opt", as defined in this specification, may be used by future PASSporT extensions as well as in conjunction with "div-o".

"opt" MUST contain a quoted full-form PASSporT, as specified by [RFC8225], Appendix A; it MUST NOT contain a compact form PASSporT. For an example of a "div-o" PASSporT containing "opt", see Section 5.

7. "div" and Redirection

The "div" mechanism exists primarily to prevent false negatives at verification services when an arriving SIP request, due to intermediary retargeting, does not appear to be intended for its eventual recipient, because the original PASSporT "dest" value designates a different destination.

Any intermediary that assigns a new target to a request can, instead of retargeting and forwarding the request, redirect with a 3xx response code. In ordinary operations, a redirection poses no difficulties for the operations of baseline STIR: when the user agent client (UAC) receives the 3xx response, it will initiate a new request to the new target (typically the target carried in the Contact header field value of the 3xx), and the "dest" of the PASSporT created for the new request will match that new target. As no impersonation attack can arise from this case, it creates no new requirements for STIR.

However, some UACs record the original target of a call with mechanisms like History-Info [RFC7044] or Diversion [RFC5806] and may want to leverage STIR to demonstrate to the ultimate recipient that the call has been redirected securely, that is, that the original destination was the one that sent the redirection message that led to the recipient receiving the request. The semantics of the PASSporT necessary for that assertion are the same as those for the "div" retargeting cases above. The only wrinkle is that the PASSporT needs to be generated by the redirecting entity and sent back to the originating user agent client within the 3xx response.

This introduces more complexity than might immediately be apparent. In the first place, a 3xx response can convey multiple targets through the Contact header field value; to accommodate this, the "div" PASSporT MAY include one "dest" object array value per Contact, but if the retargeting entity wants to keep the Contact list private from targets, it may need to generate one PASSporT per Contact. Bear in mind as well that the original SIP request could have carried multiple Identity header field values that had been added by different authentication services in the request path, so a redirecting entity might need to generate one "div" PASSporT for each PASSporT in the original request. Often, this will mean just one "div" PASSporT, but for some deployment scenarios, it could require an impractical number of combinations. But in very complex call routing scenarios, attestation of source identity would only add limited value anyway.

Therefore, STIR-aware SIP intermediaries that redirect requests MAY convey one or more PASSporTs in the backwards direction within Identity header fields. These redirecting entities will act as authentication services for "div" as described in Section 4.1. This document consequently updates [RFC8224] to permit carrying Identity header fields in SIP 300-class responses. It is left to the originating user agent to determine which Identity header fields should be copied from the 3xx into any new requests resulting from the redirection, if any; use of these Identity header fields by entities receiving a 3xx response is OPTIONAL.

Finally, note that if an intermediary in the response path consumes the 3xx and explores new targets itself while performing sequential forking, it will effectively retarget the call on behalf of the redirecting server, and this will create the same need for "div" PASSporTs as any other retargeted call. These intermediaries MAY also copy PASSporTs from the 3xx response and insert them into sequential forking requests, if appropriate.

8. Extending "div" to Work with Service Logic Tracking

It is anticipated that "div" may be used in concert with History-Info [RFC7044] in some deployments. It may not be clear from the "orig" and "dest" values which History-Info header a given PASSporT correlates to, especially because some of the target changes tracked by History-Info will not be reflected in a "div" PASSporT (see Section 1). Therefore, an "hi" element as defined here may appear in "div" corresponding to the History-Info header field index parameter value. So for a History-Info header field with an index value of "1.2.1", the claims set of the corresponding PASSporT with "div" might look like:

      { "orig":{"tn":"12155551212"},
        "dest":{"tn":["12155551214"]},
        "iat":1443208345,
        "div":{"tn":"121555551213",
               "hi":"1.2.1"} }

Past experience has shown that there may be additional information about the motivation for retargeting, which relying parties might consider when making authorization decisions about a call; see, for example, the "reason" associated with the SIP Diversion header field [RFC5806]. Future extensions to this specification might incorporate reasons into "div".

9. IANA Considerations

9.1. JSON Web Token Claims Registrations

Per this specification, IANA has added two new claims to the "JSON Web Token Claims" registry as defined in [RFC7519].

9.1.1. "div" registration

   Claim Name:  div
   
   Claim Description:  Diverted Target of a Call
   
   Change Controller:  IESG
   
   Reference:  RFC 8946

9.1.2. "opt" registration

   Claim Name:  opt
   
   Claim Description:  Original PASSporT (in Full Form)
   
   Change Controller:  IESG
   
   Reference:  RFC 8946

9.2. PASSporT Type Registrations

   This specification defines two new PASSporT types for the "Personal
   Assertion Token (PASSporT) Extensions" registry defined in [RFC8225],
   which resides at <https://www.iana.org/assignments/passport>.  They
   are:

"div", as defined in Section 3.

"div-o", as defined in Section 5.

10. Privacy Considerations

There is an inherent trade-off in any mechanism that tracks, in SIP signaling, how calls are routed through a network, as routing decisions may expose policies set by users for how calls are forwarded, potentially revealing relationships between different identifiers representing the same user. Note, however, that in ordinary operations, this information is revealed to the user agent service of the called party, not the calling party. It is usually the called party who establishes these forwarding relationships, and if indeed some other party is responsible for calls being forwarded to the called party, many times the called party should likely be entitled to information about why they are receiving these calls. Similarly, a redirecting entity who sends a 3xx in the backwards direction knowingly shares information about service logic with the caller's network. However, as there may be unforeseen circumstances where the revelation of service logic to the called party poses a privacy risk, implementers and users of this or similar diversion- tracking techniques should understand the trade-off.

Furthermore, it is a general privacy risk of identity mechanisms overall that they do not interface well with anonymization services; the interaction of STIR with anonymization services is detailed in [RFC8224], Section 11. Any forwarding service that acts as an anonymizing proxy may not be able to provide a secure chain of retargeting due to the obfuscation of the originating identity.

Also see [RFC8224], Section 11 for further considerations on the privacy of using PASSporTs in SIP.

11. Security Considerations

This specification describes a security feature and is primarily concerned with increasing security when calls are forwarded. Including information about how calls were retargeted during the routing process can allow downstream entities to infer particulars of the policies used to route calls through the network. However, including this information about forwarding is at the discretion of the retargeting entity, so if there is a requirement to keep an intermediate called number confidential, no PASSporT should be created for that retargeting -- the only consequence will be that downstream entities will be unable to correlate an incoming call with the original PASSporT without access to some prior knowledge of the policies that could have caused the retargeting.

Any extension that makes PASSporTs larger creates a potential amplification mechanism for SIP-based DDoS attacks. Since diversion PASSporTs are created as a part of normal forwarding activity, this risk arises at the discretion of the retargeting domain; simply using 3xx response redirections rather than retargeting (by supplying a "div" per Section 7) mitigates the potential impact. Under unusual traffic loads, even domains that might ordinarily retarget requests can switch to redirection.

SIP has an inherent capability to redirect requests, including forking them to multiple parties -- potentially, a very large number of parties. The use of the "div" PASSporT type does not grant any additional powers to attackers who hope to place bulk calls; if present, the "div" PASSporT instead identifies the party responsible for the forwarding. As such, senders of bulk unsolicited traffic are unlikely to find the use of "div" attractive.

12. References

12.1. Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.
   
   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              DOI 10.17487/RFC3261, June 2002,
              <https://www.rfc-editor.org/info/rfc3261>.
   
   [RFC7044]  Barnes, M., Audet, F., Schubert, S., van Elburg, J., and
              C. Holmberg, "An Extension to the Session Initiation
              Protocol (SIP) for Request History Information", RFC 7044,
              DOI 10.17487/RFC7044, February 2014,
              <https://www.rfc-editor.org/info/rfc7044>.
   
   [RFC7519]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
              (JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,
              <https://www.rfc-editor.org/info/rfc7519>.
   
   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.
   
   [RFC8224]  Peterson, J., Jennings, C., Rescorla, E., and C. Wendt,
              "Authenticated Identity Management in the Session
              Initiation Protocol (SIP)", RFC 8224,
              DOI 10.17487/RFC8224, February 2018,
              <https://www.rfc-editor.org/info/rfc8224>.
   
   [RFC8225]  Wendt, C. and J. Peterson, "PASSporT: Personal Assertion
              Token", RFC 8225, DOI 10.17487/RFC8225, February 2018,
              <https://www.rfc-editor.org/info/rfc8225>.
   
   [RFC8226]  Peterson, J. and S. Turner, "Secure Telephone Identity
              Credentials: Certificates", RFC 8226,
              DOI 10.17487/RFC8226, February 2018,
              <https://www.rfc-editor.org/info/rfc8226>.

12.2. Informative References

   [RFC5806]  Levy, S. and M. Mohali, Ed., "Diversion Indication in
              SIP", RFC 5806, DOI 10.17487/RFC5806, March 2010,
              <https://www.rfc-editor.org/info/rfc5806>.
   
   [RFC7340]  Peterson, J., Schulzrinne, H., and H. Tschofenig, "Secure
              Telephone Identity Problem Statement and Requirements",
              RFC 7340, DOI 10.17487/RFC7340, September 2014,
              <https://www.rfc-editor.org/info/rfc7340>.
   
   [RFC8443]  Singh, R., Dolly, M., Das, S., and A. Nguyen, "Personal
              Assertion Token (PASSporT) Extension for Resource Priority
              Authorization", RFC 8443, DOI 10.17487/RFC8443, August
              2018, <https://www.rfc-editor.org/info/rfc8443>.
   
   [RFC8816]  Rescorla, E. and J. Peterson, "Secure Telephone Identity
              Revisited (STIR) Out-of-Band Architecture and Use Cases",
              RFC 8816, DOI 10.17487/RFC8816, February 2021,
              <https://www.rfc-editor.org/info/rfc8816>.

Appendix A. Keys for Examples

The following EC256 keys are used in the signing examples given in this document. WARNING: Do not use this key pair in production systems.

Acknowledgments

We would like to thank Ning Zhang, Dave Hancock, Chris Wendt, Sean Turner, Russ Housley, Ben Campbell, Eric Burger, and Robert Sparks for contributions to this document.

Author's Address

Jon Peterson
Neustar, Inc.
1800 Sutter St., Suite 570
Concord, CA 94520
United States of America

   Email: jon.peterson@team.neustar

﻿